Electric incandescent lamp



March 10, 1942. 1.. J. D QIESHAL 2,275,674

ELECTRIC INCANDESCENT LAMP Filed 001;. 5, 1937 Patented Mar. 10, 1942 2,275,674 ELECTRIC INCANDESCENT LA P Leonard John Davies, Rugby, Henry William Hugh Warren, Coventry, and Gavin Hamilton, Rugby, England, asslgnors to General Electric Company, a corporation of New York Application October 5, 1937, Serial No. 167,354 In Great Britain October 5, 1936 3 Claims.

This invention relates to electric incandescent lamps and more particularly to such lamps of the type in which an incandescent filament operates in the presence of an inert gas filling, and it is an object of th invention to secure operation at an efliciency and with a life greater than has hitherto been possible.

It will be understood that the efiiciency of any normal incandescent filament lamp can be increased by simply over-running it, orincreasing its temperature, but in known lamps, any such increase of efficiency is accompanied by a corresponding decrease of life or lumen maintenance due to evaporation of the filament material and the development of hot spots etc. The use of a filling of inert gas such as nitrogen or argon or a mixture of these is known to be beneficial in preventing evaporation of the filament and it is known that as the pressure of .the filling gas is increased the beneficial effect of the filling also increases.

Lamps, however, have not hitherto been operated at pressures appreciably above 1 atmosphere and the present invention consists in providing a lamp which will operate at pressures of from 1 to 20 atmospheres or even higher.

At least two diificulties have existed up to now in connection with lamps operating at above one atmosphere.

One of these is the difiiculty of sealing off the envelope. In the normal method of removing lamps from the exhaust pumps when the internal pressure is below one atmosphere the connecting tube of glass is melted by the application of a flame or other heat and the external atmospheric pressure presses the sides or the tubes together, thus making an hermetic seal. If however, the pressure within the lamp when sealing oil is performed is greater than atmosphere, then the exhaust tube walls are blown out instead of collapsing and a seal cannot be made- The second difliculty is connected with the safety of operation of the lamps. If a lamp of normal size is intended for operation at an internal pressure of say 2 to 5, or more atmospheres, then in order to secure freedom from bursting, owing to large unsupported areas of the bulb walls, it is necessary to make the walls of a considerable thickness of glass. Even so the large volume of compressed gas represents a considerable amount of energy and the lamp may burst in transit or in service and the thick glass constitutes a danger.

We propose to overcome these difilculties in the following novel ways.

A lamp intended for operation at a pressureof 1 to 20 atmospheres or more is made much smaller than normally. For example, for a lamp consuming say 80 watts the size may vary from a diameter of 5 to 12 mms. and a length of 20 to 25 mms. for a quartz-envelope to a larger size of say, 7 mm. diameterand 35 mms. long for a hard glass envelope. Owing to the small volume of the lamp, the energy released, should the lamp burst, is small, and owing to the small size any unsupported span of bulb wall is relatively short and consequently the wall thickness need not be unduly great. We have found, for example, that quartz tubing 1 to 2 mms. wall thickness and 5 mms. bore will withstand at a temperature of approximately 1000" C. an in-.

ternal pressure of more than 20 atmospheres.

By adopting a small size, and using a material such as quartz or special glass to withstand the high temperature a construction is provided that not only is easier to construct to resist bursting, but should a fault develop, is less dangerous than a burst occurs.

With regard to the difficulty of. seal off, at,

pressures higher than atmospheric, this may be overcome as follows:

In one form the lamp is made with a filling of mercury as the inert gas. During the manufacturing process in which the lamp is sealed off from the exhaust system, the mercury vapour pressure is low, and much below 1 atmosphere,

since the lamp temperature is low. In operation, however, the mercury pressure reaches the desired value owing to the rise in temperature 7 of the lamp.

In order to secure a definite operating pressure, largely independent of small variations of bulb size, shape and ambient temperature, the amount of mercury placed within the lamp is controlled, and the lamp shape is controlled so that at some value, for example of the operating wattage, the mercury is completely vaporised.

- The pressure at operating wattage will then be controlled by the milligrammes of mercury per cc. of volume of the lamp.

Two difilculties are associated with the use of mercury, both connected with the-low pressure when the filament is cold. At this stage the pressure of mercury vapour-is below 1 atmosphere and a certain time must elapse after switching on before the mercury reaches the desired operating pressure. During this time the filament is operating at a high temperature in a low pressure, and consequently may suffer serious evaporation. Also at a low pressure of mercury vapor an arc may occur between the ends or other portions of the filament, with destructive or undesirable effects. These consequences may be avoided by arranging a separate heater .filament around the quartz lamp and a time operating temperature in less, say than 2 to 5 seconds. To further resist any tendency to evaporation and/or arcing during the first portion of the warming up period, the lamp may contain a pressure of permanent gas such as argon or nitrogen or krypton or a mixture of such gases at about. 300-600 mms. in addition to the controlled quantity of mercury.

Alternatively the lamp may contain argon, ni-

trogen or other inert gas or a mixture of these at a pressure of one toten atmospheres or more and be fitted with a metal exhaust tube. The sealoi'l is then performed by pressing the side of the metal together and then electrically welding these, preferably using a heavy current short duration spot weld.

The metal seal-011 tubernay be attached to the lamp by means of a metal to glass seal. For example a metal of the type known by the trade name of Femico which has a coeflicient of expansion substantially the same as glass may be used with a borosilicate glass. If'desired the lamp body may be made of-a glass sealing to this or may be made of quartz with a graded. glass seal. The metal exhaust tube may be of steel electrically welded to a. Fernico or other alloy part velope; and Fig. 2 is a similar view of a modification in the form of a double-ended lamp with a longitudinally extending coiled filament, the metal seal-off tube at one end being shown as it appears prior to sealing or closure thereof.

Referring further to the drawing, the translucent envelope ll, filament l2, and metal sealoif tube I3 are shown in assembled relation. The said envelope contains a predetermined quantity of mercury H which, as fully described above, is completely vaporized during operation of the lamp. In Fig. I, the sides of the metal seal-off tube l3 are shown pressed together and welded while, in Fig. 2, the seal-ofi tube I3 is illustrated as it might appear before the sides are pressed together. i

What we claim as new and desire to secure by Letters Patent of the United-States is:

1. An incandescent electric lamp comprising a light-transmitting envelope of high melting point vitreous material having a filament mounted therein and containing a quantity of mercury and a filling of an inert gas, the amount of mercury taken with the pressure of the inert gas being such that at the operating temperature of the lamp the mercury is completely vaporised and the pressure of the combined gas and mercury vapour exceeds one atmosphere.

2. An incandescent electric lampcomprising a light-transmitting envelope of high melting point vitreous material having a filament mountwhich in turn is sealed to the glass envelope.

The filament-leads may becarried by the metal part or the glass envelope.

In the appended drawing, Fig. 1 is a schematic elevation, partly in section, of a single-ended lamp comprising our invention and having a compact coil filament in the envelope with a closed metal seal-off tube at one end of the said en-' ed therein and containing a filling of an inert gas at a pressure of about 300 mms. and also such a quantity of mercury that at the operating temperature of the lamp the mercury is completely vaporised and the combined pressure of the gas and vapour exceeds one atmo phere.

3. An incandescent electric amp comprising a light-transmitting envelope of high melting point vitreous material having: a filament mounted therein and containing a filling of an inert gas at a pressure of less than one atmosphere and also such a quantity 0? mercury that at the operating temperature 0 the lamp the mercury is completely vaporized and the combined pressure of the gas and vapour exceeds one atmosphere. I

LEONARD JOHN DAVIES.

HENRY WILLIAM HUGH WARREN. GAVIN HAMILTON. 

